Wireless LANs (WLANs) are defined as wireless networks that provide inexpensive, high-speed, wireless communications in office buildings, the home, public areas, and a variety of commercial and industrial locations. A “STAtion” (STA) in a WLAN can be a personal computer, a bar code scanner, or other mobile and/or stationary devices with the appropriate integrated chip set or wireless network interface card to provide the necessary connection over a wireless link to other stations. WLANs generally operate at peak speeds ranging between 1 and 54 Mbps, and have a range of between 15 and 300 meters.
Wireless LANs can provide wireless peer-to-peer communication between stations (defined as an “ad-hoc” group), as well as access to a conventional “wired” network. A single-cell WLAN using an Access Point (AP) may serve a group of stations communicating over the wireless medium, while connecting them to an external wired network. Alternatively, multi-cell WLANs provide greater range than single-cell WLANs with multiple access points interconnected by means of a wired network. An access point can be thought of as the counterpart to a base station of a mobile cellular communication system and functions as the gateway to the (wired) distribution system.
Conventional wireless LANs operate in the unlicensed portion of the spectrum, where they provide interference-free simultaneous transmission on multiple channels, each cell transmitting on a single (different) channel. The number of channels available varies with the spectrum allocation and physical layer technology. For example, the IEEE 802.11b standard provides three TDD channels for duplex data transmission at speeds up to 11 Mbps in the 2.4 GHz ISM band, while the IEEE 802.11a standard provides eight channels at speeds up to 54 Mbps in the 5 Ghz band. In some environments, however, the number of channel frequencies available is insufficient to allow each cell to operate without experiencing co-channel interference. This interference limits the quality (i.e., increases the packet error rate) of transmission to stations.
Although wireless LANs have traditionally been used for data sessions only, where errors can be overcome by retransmission of packets, the growth of multimedia content is inducing a desire for so-called “streaming” support as well. Real-time streaming applications must deliver packets without any accumulation of delays, since delay renders packets useless for reconstitution of source waveforms. An example of a streaming application is a WLAN Voice-Over-IP (VoIP) phone, for example, an IEEE 802.11b phone, that uses the WLAN to convey voice telephony. The WLAN communicatively couples the IEEE 802.11b phone to other phones across the Internet, other IEEE 802.11b phones, and/or to other phones via various communication paths.
Aside from delays due to errors caused by interference, current arrangements using a WLAN to support voice communications can also be impaired by “contention” when multiple stations attempt to simultaneously transmit their packets. If the WLAN attempts to service both voice and data communications, the WLAN may not have sufficient capacity (due to the shared nature of the channel) to hold latency below that necessary to prevent interruptions or artifacts in the telephony streams. Other services, such as video conferencing and real-time streaming IP video, can experience similar difficulties. Moreover, roaming within a WLAN (between APs) can introduce significant gaps in service, where such gaps also violate the low latency requirements of real-time streaming communication.
Additionally, wireless LANs were originally conceived for primarily indoor operation, with cell sizes on the order of 100 feet (or less) in radius. Although outdoor operation can extend the cell size to perhaps a radius of 300 feet (with a lowered transmission rate), wireless LANs that attempt to provide both indoor and outdoor coverage experience poor link quality due to the difficulty of propagating RF signals through building walls (which are known to significantly attenuate the signal).
Attempts to use outdoor/indoor single-tier (i.e., single RF link from wired network to station) wireless LAN architectures for service provider local access applications are thus challenged by these issues of signal quality, co-channel interference and the inability to provide deterministic Quality-of-Service (QoS), as particularly required for voice/streaming video applications.